Optical scanning systems find use in a variety of applications such as thermal imaging systems known as FLIR's (Forward Looking Infra Red systems). In such systems thermal radiation, for example, in the infrared range, is typically collected by a lens and the image is raster scanned in the vertical and horizontal directions. The radiation from each picture element in succession is focussed onto a radiation detector which provides an electrical signal according to the detected radiation intensity. The electric signal from the detector can then be used to create a television-like video display corresponding to the original thermal image.
Although the invention herein will be described principally in connection with thermal imaging systems, the invention is not limited thereto. In particular, it should be noted that the invention is also operable with respect to other types of radiation such as light in the visible spectrum. Furthermore, although the principal description of the system will be in the "read" mode, the system is also capable of operating in the reverse direction to paint or "write" an image starting from a variable intensity control signal.
In the past optical scanning has been achieved in a variety of different ways usually employing moving mirrors to provide the horizontal and vertical scanning. In its simplest form, two separate oscillating mirrors are employed, one rotating about an axis so as to provide the horizontal scan and the other rotating about an axis so as to provide a vertical scan. The oscillating motions of the mirrors rotate the mirror in one direction to provide the scan followed by a rapid "flyback" rotation to position the mirror at the beginning of the next scan. The oscillating motion of the high speed horizontal scanning mirror imposes severe limitations on the operable scanning speed with this arrangement. This approach also suffers from scanning distortions of the image and electronic signal processing difficulties.
A common approach utilized to improve the scanning speed is to replace the high speed horizontal oscillating mirror with a multi-faceted rotating polygonal mirror. With this arrangement successive facets of the polygonal mirror sweep the image to provide the horizontal scans. The facets can either be on the outside of a solid polygon (see for example, U.S. Pat. Nos. 4,210,810, 4,180,307 and 4,156,142) or on the inside of a cylinder (see for example, U.S. Pat. No. 3,604,932). Since the high speed horizontal scanning mirror motion is rotary rather than oscillating, higher scanning rates are possible.
However, in order to operate such polygonal mirrored systems at television raster scanning rates (15,750 Hertz or 63.5 microseconds per line) the rotating speed for the polygonal mirror is typically in the range of 40,000 to 80,000 rpm. Such high operating speeds result in critical motor design problems, particularly in handling the high frequency motor energizing signals, in precisely balancing the rotating apparatus, and in achieving reasonable bearing lift. Normally, in such high speed systems the rotating polygon mirror structure must be placed in an evacuated chamber to reduce air resistance, hence, adding considerably to the cost and operating difficulties of the system.
The number of facets in a polygon mirrored system is determined, generally, by the desired scanning rate and the available motor speed. The mirror surfaces must be large enough to accommodate the optical pupil diameter at the entrance to the system and, hence, the mirror structure cannot be artitrarily small. The mirror surface is preferably large enough to avoid serious vignetting whereby part of the incoming image misses the mirror at positions toward the ends of the horizontal scan lines. In a system designed to achieve quality imaging the rotating mirror structure is of a considerable size imposing considerable load on the drive motor system.
Another problem encountered with polygon mirrored systems is their poor scanning efficiency. In a scanning system it is desired to devote maximum time to scanning the image with a minimum amount of lost time between successive scans. In a polygon mirrored system the scanning efficiency is relatively low, for example, on the order of 25 percent for a six sided polygon scanning a 30 degree field of view.
A further problem with polygon mirrored systems arises when used in combination with telescopes. Normally there is insufficient distance between the telescope pupil at the entrance to the system and the first element of the imaging lens in which to accommodate the horizontal and vertical scanning mirrors. In order to increase the available space for the mirrors it becomes necessary to add relay or transfer optics to the system. These undesirably add size, weight and cost to the system.
Although rotating polygon mirrored systems as discussed above are the most common of the prior optical scanning systems, two further approaches should be mentioned. One such other approach utilizes a star shaped mirror wheel which is shaped like a gear wheel whereon the scanning mirrors are located on the sides of the gear teeth. For example, see the IR Handbook, by ONR Department of Navy, Library of Congress NO. 77-90786 page 10-23. This approach tends to reduce the required rotating speed for scanning horizontal traces and eliminates some of the optical problems but, on the other hand, suffers from degraded imagery due to focus shifts inherent in the operation and due to difficulties in maintaining the required surface tolerances for the star wheel configuration.
Another known approach is to use a linear array of detectors capable of sensing radiation intensity along the entire horizontal trace thereby eliminating the need for one of the scanning mirrors. The horizontal detector arrays however, are too expensive for most applications and usually result in undesirable configurations for the scanner unit.